We present a theoretical study on the electronic structure of four periodic B-DNA models labeled as (AT) 10 , (GC) 10 , (AT) 5 (GC) 5 , and (AT-GC) 5 with A=Adenine, T=Thymine, G=Guanine, and C=Cytosine. Each model has ten base pairs with Na counter-ions to neutralize the negative phosphate group in the backbone. The (AT) 5 (GC) 5 and (AT-GC) 5 models contain two and five (AT-GC) bilayers respectively. When compared against the average of the two pure models. We have estimated the (AT-GC) bilayer interaction energy to be 19.015 Kcal/mol, which is comparable to the hydrogen bonding energy between base pairs obtained from the literature. Our investigation shows that the stacking of base pairs plays a vital role in the electronic structure, relative stability, bonding, and distribution of partial charges in the DNA models. All four models show a HOMO-LUMO gap ranging from 2.14 to 3.12 eV with HOMO states residing on the (PO 4 + Na) functional group and LUMO states originating from the bases. Our calculation implies that the electrical conductance of a DNA molecule should increase with increased base-pair mixing. Interatomic bonding effects in these models are investigated in detail by analyzing the distributions of the calculated bond order values for every pair of atoms in the four models including hydrogen bonding. The counter-ions significantly affect the gap width, the conductivity, and the distribution of partial charge on the DNA backbone. Also evaluated quantitatively are the surface partial charge density on each functional group of the DNA models.